There are many examples of valuable proteins that are useful in pharmaceutical and industrial applications. Often these molecules are required in large quantities and in partially or highly purified formulations to maintain product quality and performance. Plants are an inexpensive source of proteins, including recombinant proteins. Many have proposed the desirability of producing proteins in large amounts in plants. However, the problems associated with extracting and processing products from homogenized plant tissues as well as purifying and recovering the recombinant protein product have been recognized as substantial. Austin et al. Annals New York Academy of Science, 721:234–244 (1994). These problems represent major impediments to successful recombinant protein production in plants on a large and commercially valuable scale.
Plant cells are thought to synthesize proteins on the membranes of the endoplasmic reticulum and transport the proteins synthesized to the cell surface in secretory vesicles formed at the Golgi apparatus. A discussion of the topic is provided by Jones et al., New Phytology, 111:567–597 (1989). Significant research has been devoted to elucidating the specific mechanisms related to protein secretion for several particular proteins in specific plant tissues or cell cultures. Examples of such efforts are presented by Herbers et al., Biotechnology 13:63–66 (1995), Denecke et al., The Plant Cell 2:51–59 (1990), Melchers et al., Plant Molecular Biology 21:583–593 (1993) and Sato et al., Biochemical and Biophysical Research Communications 211(3):909–913 (1995). In the case of proteins not secreted into the plant cell apoplasm or intercellular space, a mechanism for lysing the plant cell wall must be utilized in order to release and capture the protein of interest. Plant cells must be exposed to very high shear forces in order to break the cell walls and lyse cellular membranes to release intracellular contents. Proteins of interest, whether recombinantly produced or naturally produced by the subject plant, are thereby exposed to a hostile chemical environment and are particularly subject to oxidative and proteolytic damage due to the exposure of the product to enzymes and small molecules that were compartmentalized before homogenization of the tissue. In addition, most of the other total cellular protein is mixed with the protein of interest creating formidable purification problems if such a cell lysis procedure is performed. In order to use the biosynthetic capacity of plants for reliable protein production, a process to obtain specific proteins that can be secreted into the intercellular space (apoplasm) of plant tissues is desirable. Such a procedure would forego the need for homogenization. If such a procedure is performed, the fraction of plant material containing one or more proteins of interest might be obtained without homogenization. Therefore, such a procedure provides that the plant extract is enriched for the particular protein of interest, and the protein is protected from some chemical and enzymatic degradation.
Since the valuable proteins and products of interest are partitioned or secreted into the interstitial spaces, vacuum pressure facilitates the introduction of infiltration medium into the interstitial space. Similarly, various forces can be applied to remove the retained fluid. Centrifugal force of 1,000×G is effective. Using gravity, the retained fluid can be collected in a trap under vacuum. With or without vacuum infiltration of a buffer, the enzyme can be recovered by freezing the tissue, thawing and applying a physical press to recover the fluid. However, such a procedure results in an undesirable increased cellular lysis.
Genetically modified plants are a reliable source for the production of recombinant proteins. Because the biological product is accumulated under nonsterile growth conditions and the production may be scaled to the quantities desired in a relatively inexpensive manner, it is feasible to exploit a dilute but enriched source such as the interstitial fluid fraction as a S source for harvesting proteins of interest on an industrial scale. A variety of proteins of interest may be harvested from recombinant plant sources, however, highly active, pharmaceutical quality enzymes, cytokines and antibodies are particularly valuable products that can be developed by this process.